An electrolyzer is defined as an apparatus where an electrolysis reaction takes place. Electrolysis is the process of decomposing a chemical compound into its elements or producing a new compound by the action of an electrical current. Basically, an electrolyzer is composed of two electrodes and a separator called a membrane. In the Chlor-alkali industry, primary products of electrolysis are chlorine, hydrogen, and sodium hydroxide solution (commonly called “caustic soda” or simply “caustic”). Three main electrolysis processes are used in the Chlor-Alkali industry: membrane, diaphragm and mercury. Because of the growing environmental concerns the latter processes are being replaced the membrane electrolysis process. In the chlorate industry, sodium chlorate or sodium hypochlorite is produced from the electrogenerated chlorine and caustic with no separator in the electrolysis cell. Fuel cells where water is electrolysed to produce hydrogen are also in the background of the present invention.
FIG. 1 identified as Prior Art is a schematic representation of a typical membrane cell used in the Chlor-alkali industry. It is composed of two compartments. The anode compartment is fed-up with a saturated brine solution (NaCl) while a dilute caustic soda passes through the cathode compartment. In the chlor-alkali plants, chlorine is generated at the coated (usually Ti) anode 2. The combination of hydroxide ions with migrated sodium ions across the selective membrane 1 generates caustic soda (NaOH) and Hydrogen gas. The cathode 3 is usually nickel with a catalytic coating to reduce the overpotential for H2 evolution. The complete chlor-alkali process is described by the following equation:2NaCl+2H2O→Cl2+H2+2NaOH
Commonly in the recent chlor-alkali production plants, an electrolyzer is defined as a combination of elementary membrane cells. The electrolysis process takes place in each cell after applying a current. Therefore, the electrolyzer energy consumption plays a key role in the process. The electrolyzer overall performance is mainly related to each cell efficiency. It is well known in the art (“A First course in Electrode Processes”, Derek Pletcher, “Ion Permeable Membranes”, Thomas A. Davis, J. David Genders, Derek Pletcher), that voltage variations in the membrane cell are generally a result of physical changes within the cell components. The cell voltage variation is distributed between its components: anode, cathode, membrane and electrical connections. An abnormal decrease or increase in the cell voltage is generally considered as a premise to potential problems.
Known in the art is the article entitled “A Simple Procedure for Evaluating Membrane electrolyzer Performance” by K. L. Hardee in Modern Chlor-Alkali Technology V.6 pp. 234 1995. The author proposes to use curve-fitting coefficients to diagnose an elementary cell. This publication's focus was on the extraction of the fitting coefficients and their use to characterize the cell parts. However, it doesn't cover the aspect of the coefficients quality and the automation process for an electrolyzer composed of large number of membrane cells. The voltage and current data collected from operating plants are not always suited enough for the curve fitting procedure. Therefore, one aspect of the present invention is to propose analytical methods for the extraction of good curve fitting coefficients and a procedure for the classification of those parameters.
In accordance with above mentioned work and other known publications (“Voltag-Current curves: Application to membrane cells”, D. Berger, M. Hartmann and H. Kirsch, Modern Chlor-Alkali Technology Vol. 4, Chap. 15) each elementary cell voltage can be approximated by an equation of the form:Ucell=U0+S×log(I)+R×I  Equation 1
Thus after assuming that the electrodes follow a Tafel behaviour, i.e. are not mass transport limited, and that the other voltages are due to ohmic resistances, we could detail each term as follow:U0=Aa+Ac+Ea+Ec R=Re+Rs+Rb S=Ba+Bc Where:Ba: The anode Tafel slopeAa: Anode Log of the exchange current densityEa Anode Equilibrium PotentialBc: The anode Tafel slopeAc: Anode Log of the exchange current densityEc: Anode Equilibrium PotentialRe: Electrolyte resistanceRm: Membrane resistanceRs: Structure/contact resistanceI: Current density
The (S,R,U0) coefficients evaluation method proposed within the present invention will give ultimately a clear idea on which part of the cell is failing: membrane (R), electrodes(S,U0), electrolyte or the cell structure (R).
Schetter Thomas in patent application DE10217694 describes a method for dynamic determination of the voltage-current characteristic curve of a fuel cell during operation under different loading conditions. Although this document addresses the problem of extracting voltage-current linear curve parameters, it doesn't bring a useful method for analyzing these parameters in an industrial scale and relate them to cell performance.
In industrial electrolysis processes, a non-negligible consideration is given to the energy consumption, which is directly related to the cell performance and to the process current efficiency. The most important issues that affect the cell performance are: the current efficiency of the two products (Cl2, NaOH), their purity, the membrane resistance and its lifetime and finally the electrodes' activity (coating). While the membrane resistance and electrodes' activity could be characterized and evaluated by the curve-fitting method described in the present invention, the products current efficiency diagnosis and optimization is a more complex issue.
It is well known in the art (“A First course in Electrode Processes”, Derek Pletcher) that the energy consumption is proportional to current efficiency as following:
                              Energy          ⁢                                          ⁢          consumption                =                  -                                    n              ×              F              ×                              E                cell                                                    3.6              ×                              10                3                            ×              ϕ              ×              M                                                          Equation        ⁢                                  ⁢        2            where    n: Number of cells in the electrolyser    F: Faraday constant    Ecell: Cell Voltage    φ: Fractional current efficiency    M: Molecular weight of the product in kg.
According to known studies, a major reason for the loss in the current efficiency is the back-migration of hydroxide ions through the cation exchange membrane from the catholyte to the anolythe and also the membrane conductivity. The hydroxide ions back-migration is mainly due to the sodium hydroxide (NaOH) strength. The increase in the membrane conductivity results from a change in the electrolyte temperature.
Howard L. Yeager and Adam A. Gronowski in “Factors which influence the Permselectivity of High Performance Chlor-Alkali Membranes” outline the influence of sodium hydroxide concentration on the current efficiency for a laboratory Nafion™ bilayer membrane cell. This study sets forth a theoretical complex relationship between the two parameters. Thomas A. Davis, J. David Genders, Derek Pletcher in “Ion Permeable Membranes” also indicate a relationship between the membrane conductivity, the brine impurities and the current efficiency for a Nafion™ membrane cell.
All the aforementioned studies were done in a laboratory-controlled environment where it is easy to extract the current-voltage curve characteristic, while in a real operation plant the measurements are often not reliable due to control problems. Thus, the aim of one aspect of the present invention is the online generation of relationship between current efficiency and operational measurements such as the sodium hydroxide and catholyte temperature.